As multimedia applications have grown in popularity and technology relating to the Internet and wireless technology has advanced, there has been a rapid rise in the use of digital media such as images. For example, numerous images are now accessible through the World Wide Web. However, because of the accessibility of these digital images, they can be easily downloaded and used by an unauthorized user for an unexpected purpose. As a result, digital watermarking techniques are often used to prove ownership and authenticity of digital media by hiding information, such as a random sequence or a logo, into the media.
To enhance the security of a watermark in an image, it can be appreciated that the watermark should be embedded in the image in such a way that it is perceptually transparent and robust. However, most existing image watermarking techniques are lossy and introduce permanent distortion into a host image during the embedding process, which results in a permanent peak signal-to-noise ratio (PSNR) loss in the host image. As a result, many existing watermarking techniques cannot be applied in medical, military, and/or applications that are sensitive to embedding distortion and prohibit permanent loss of signal fidelity. A lossless, reversible watermarking scheme is therefore required for such applications, where the original host signal can be perfectly recovered upon extracting the watermark from the host signal. Algorithms for reversible watermarking have been proposed; however, the payload of such algorithms is prohibitively small as compared to that achievable by lossy watermarking algorithms.
Further, in the specific example of applications such as books, magazines, newspapers, printer outputs, and fax documents that utilize black-and-white halftone images that contain only black and white pixels, it is often desirable to hide value-adding invisible data, such as a company logo, for copyright protection and/or authentication purposes. However, most existing data hiding algorithms cannot readily be applied to embed data into these and/or similar halftone signals.
In view of the above, there exists a need for efficient and high-payload watermarking techniques for digital images.